Polymeric materials are well known and widely used because of their high versatility. Polypropylene is a particularly desirable polymer because of its low cost compared to engineered plastics and because it is highly amenable to recycling. One particular drawback of polymers is that because they are hydrocarbon-based, they are generally not fire resistant.
It is known to make polymeric materials more fire resistant or fire retardant by the use of additives. However, the use of additives can compromise other qualities of the polymer, such as for example the mechanical strength.
Some additives provide an appropriate balance between fire resistance and mechanical strength, however many of these additives are undesirable from point of view of cost or chemical toxicity. For instance, the most efficient flame retardant additives currently available are brominated materials, such as the decabromodiphenyl ethers (DecaBDEs). DecaBDEs have been used as flame retardants in electronics, wire and cable insulation, textiles, automobiles and aircraft, amongst others. Although DecaBDEs are very good at minimising the spread of flame, they are not recyclable and in addition have highly toxic and carcinogenic degradation products. DecaBDEs have been banned in many places including Europe and the US, where it is expected that they will be phased out completely by the end of 2013.
Other additives used to combat the spread of fire in polymeric materials include metal hydroxides such as alumina trihydrate (ATH) and Mg(OH)2 or materials such as halloysite nanotubes (HNT). These have been used to impart fire retardancy properties to polymeric materials, however they require a very high dose, usually greater than 50% w/w, in order to achieve a high (UL-94 V-0) fire rating. They have a low level of compatibility with the polymer matrix and the resultant material typically has a very poor appearance as well as low gloss and scratch resistance.
It is an object of the present invention to provide a fire retardant material including polypropylene which has both high fire retardancy rating and a good level of flexural and tensile modulus.
According to a first aspect the invention provides a fire retardant material comprising polypropylene having a flexural modulus of 2.8 GPa or above and a fire retardancy rating of UL-94 V-1 or better.
Alternatively, the fire retardant material may have a flexural modulus of 3 GPa or above and a fire retardancy rating of UL-94 V-1 or better.
Alternatively, the fire retardant material may have a flexural modulus of 2.8 GPa or above and a fire retardancy rating of UL-94 V-0.
Preferably the fire retardant material has a flexural modulus of 3 GPa or above and a fire retardancy rating of UL-94 V-0.
The fire retardant material is preferably halogen-free and/or free from glass-fibres.
Preferably, the fire retardant material comprises at least 60% polypropylene, more preferably 60-80% polypropylene and most preferably at least about 70% polypropylene.
In the present invention, any form of polypropylene can be used. Isotactic or syndiotactic polypropylene or any mixture thereof, or of any degree of crystallinity can be used. The polypropylene may be initially in pelletised form, or it may be all or partially in powdered form. The fire retardant material further includes a fire retardant, preferably in an amount of 10-35%, more preferably in an amount of 15-25% and most preferably in an amount of about 20%.
Preferably the fire retardant is an ammonium polyphosphate (APP), more preferably a polymeric APP-crystal phase II type fire retardant.
The preferred fire retardant of the present invention is a polymeric APP-crystal phase II type fire retardant. APP fire retardants are based around ammonium polyphosphates of the following structure.

The APP II structure is cross-linked or branched, with “n” being typically higher than 1000. Polymeric APP-crystal phase II type fire retardants have high stability and low water solubility.
Most preferably the fire retardant agent is Budit®3167. Budit®3167 is a commercially available fire retardant. Budit®3167 is of low extractability in water compared to other traditional fire retardants and is thus considered to be an environmentally friendly halogen-free alternative. Budit®3167 contains all active components necessary for intumescence: catalyst, carbonific and spumescent agent. Budit®3167 is also regarded as a highly desirable smoke retardant from the point of view of its low smoke density and low smoke toxicity upon burning. Budit®3167 satisfies the stringent requirements for use in aircraft interiors, with the limits of hydrogen cyanide, carbon monoxide, nitrous gases, sulphur dioxide, hydrogen fluoride and hydrogen chloride all falling well below the limits specified by the ABD (Airbus Directive) 0031 test.
The fire retardant material further includes a filler, preferably talc. The talc is preferably present in an amount of 5-20% wt/wt, more preferably 8-12% wt/wt and most preferably in an amount of about 10% wt/wt to impart higher stiffness to the fire retardant material.
Talc is a hydrated magnesium silicate whose chemical formula is Mg3Si4O10(OH)2 with flattened tabular crystals with a hexagonal cross-section. Talc is present as a chemically inert filler, a reinforcing agent, a heat sink and a nucleating agent to provide improvements in the crystallisation temperature (i.e. raising of the freezing point of the polypropylene) as measured by differential scanning calorimetry (DSC). An improved crystallisation temperature with a consequently lower heat flow translates into a high heat deflection temperature, greater control of warpage, shrinking and other dimensional elements and may further provide improvements in injection moulding cycle times to enable parts to be removed more quickly from the mould.
It is known that talc consists of a layer or sheet of brucite (Mg(OH)2) sandwiched between two sheets of silica (SiO2). The layers of Mg(OH)2 are bonded by weak van der Waals' forces and hence talc can readily undergo exfoliation and cleavage to form high aspect ratio particles, which can significantly improve the stiffness of polymers. Milling is a useful way to achieve high aspect ratio particles.
The fire retardant may further include an antioxidant, for example, Irganox® 1010. The antioxidant may be present in an amount up to 3% wt/wt, more preferably in an amount up to 1% wt/wt.
According to a second aspect the invention provides a fire retardant material comprising: polypropylene 60-80%;
fire retardant 10-30%; and
talc 5-20%
According to a third aspect the invention provides a fire retardant material comprising:
polypropylene 60-70%;
fire retardant 20%; and
talc 5-20%
According to a fourth aspect the invention provides a fire retardant material comprising:
polypropylene 60-80%;
a polymeric APP-crystal phase II type fire retardant 10-30%; and
talc 5-20%
According to a fifth aspect the invention provides a fire retardant material comprising:
polypropylene 60-70%;
a polymeric APP-crystal phase II type fire retardant 20%; and
talc 5-20%
The present applicants have surprisingly found that a polypropylene having a specific combination of fire retardants and prepared in a specific manner displays suitable levels of mechanical strength and fire retardancy.
According to a sixth aspect the invention provides a method of forming a fire retardant material comprising:
forming a first blend of polypropylene and filler;
forming a second blend of polypropylene and fire retardant
combining the first and second blends; and optionally adding polypropylene
According to a seventh aspect the invention provides a method of forming a fire retardant material comprising:
forming a first blend of polypropylene and filler;
combining the first blend with a fire retardant; and optionally adding polypropylene
Preferably, total polypropylene is 60-80%, more preferably at least 60% and most preferably at least 70%.
Preferably total fire retardant is 10-35%, more preferably 15-25% and most preferably 20%
Preferably, the fire retardant is an ammonium polyphosphate, more preferably a polymeric APP-crystal phase II type fire retardant, most preferably the fire retardant agent is Budit®3167.
Preferably the filler is talc, which is preferably present in an amount of 5-20% wt/wt, more preferably 8-12% wt/wt and most preferably about 10% wt/wt.
The method may also further include the step of adding an antioxidant, such as Irganox®1010. The antioxidant may be present in an amount up to 3% wt/wt, or more preferably in an amount up to 1% wt/wt.
The method of combining the chemical species in the present invention is of considerable significance. The method of mixing requires that the talc is added in a “masterbatch” form. That means the talc is pre-mixed with the polypropylene prior to the addition of the fire retardant, or a pre-prepared commercial blend of polypropylene and talc is used. Without wishing to be bound by theory, it is believed mixing in this order minimises interference between the talc and the fire retardant. It is believed fire retardants such as Budit®3167 can become trapped or bonded to the magnesium hydroxide in between the silica plates in talc. By preparing the masterbatch of the polypropylene with the talc, this subsequent deactivation of the fire retardant is avoided.
FIG. 1 shows the X-ray powder diffraction pattern of talc and talc in a number of PP blends with Budit 3167 of the present invention, illustrating exfoliation of talc in the PP Budit 3167 blends.
Scanning electron microscopy (SEM) studies indicate the commercially available heat stabilized crystal phase II APP fire retardant additives disappear from the PP matrix in the presence of traditional fire retardant additives like Mg(OH)2 indicating a possible reaction with APP and diminishing the effectiveness of the fire retardant additive. Similar effects were observed in the presence of nanotubes such as halloysite. However the presence of an inert filler like talc resulted in no interference to the effectiveness of the fire retardant additive. FIG. 2 shows a series of SEM images which illustrate the dispersion of the fire retardant in the polypropylene matrix, the exfoliation of talc and, in some cases, the disappearance of the fire retardant in the presence of unsuitable fillers.